Filter material

ABSTRACT

A non-woven cellulose ester fibrous filter sheet material comprising cellulose ester staple fibers and cellulose ester fibrets which exhibits improved filtration properties and the method of its preparation.

This is a continuation of application Ser. No. 937,228, filed Aug. 28,1978, now abandoned, which is a continuation of application Ser. No.730,039 filed Oct. 6, 1976, now abandoned.

The present invention relates to a sheet-like structure suitable for useas a filter material and more particularly to a sheet-like structuresuitable for filtering tobacco smoke. It further relates to filterswhich will efficiently remove the harmful constituents of tobacco smoke,in particular tar and nicotine, without objectionably increasing thedraw to a high level. The present invention also relates to processesfor making such sheet-like structures and filters.

The principle use contemplated for the material of this invention is ina filter for removal of respirable particles of any kind as well asliquid filtration. When employed as a filter for the removal ofrespirable particles, the filter of this invention may be used inconjunction with cigarette or other smoking articles such as a pipe,cigar or cigarette or cigar holder. It will be obvious from thedescription which follows, however, that the filter material of thisinvention may also be advantageously utilized in surgical masks, for airfiltration and in face masks for smog or dust protection.

Many types of filter materials have been proposed, particularly fordecreasing the amount of harmful ingredients of tobacco smoke reaching asmoker's respiratory system. However, in addition to removing a highproportion of the harmful ingredients, a satisfactory filter must alsofunction without unduly impeding the passage of air or smoke through thefilter so as to result in too high a draw. When used in filteringtobacco smoke, the filter material must also not distort the taste ofthe tobacco smoke by adding a taste of its own. Another factor in theproduction of a satisfactory tobacco filter is that it must be capableof inexpensive fabrication so as not to make the ultimate price of thesmoking article with which it is used too costly.

While a wide variety of fibrous materials have been conventionallyemployed as filter material, only wood pulp fibers and cellulose acetatehave met with any significant commercial acceptance.

Wood pulp fibers are usually used in the form of a paper which iscorrugated and/or condensed into a rod form for attachment to acigarette while cellulose acetate is conventionally used in the form ofa tow of substantially longitudinally extending continuous filamentswhich are preferably crimped to cause short sections of the individualfilaments to run at random in non-parallel diverging and convergingdirections to the predominant longitudinal direction of the tow.

Conventional paper filters are generally characterized by higherfiltration as measured by smoke removal efficiency, but also adverselyaffect taste and odor of the delivered smoke stream. Moreover, theirphenol selectivity is significantly lower than that available when usingconventional cellulose acetate tow filters. Further, paper filters aresusceptible to collapse during smoking, primarily because of theirtendency to absorb moisture from the tobacco smoke stream and smoker'smouth. Also, the compressibility of paper filters at a given pressuredrop is generally greater than that of conventional tow filters ofcomparable weights.

In comparison with paper filters, conventional cellulose acetate towfilters overcome all the disadvantages of paper filters recited abovewhile admirably meeting the requirements of good draw and economy. As aresult, the major proportion of filter cigarettes now on the marketutilize this type of material in spite of the fact that celluloseacetate tow filters exhibit smoke removal efficiencies at a given drawthat are relatively lower than that of paper filters.

Many types of filters have been suggested to overcome this disadvantagewhile maintaining the advantages of cellulose acetate and therebyattaining a filter having the attributes of both filters in one filterwhich exhibits high smoke removal efficiencies and an acceptable draw. Acommonly utilized concept to accomplish this end is the combination ofcellulose acetate tow and paper in a single filter. This has beenaccomplished by the juxtaposition of a short rod of paper and a shortrod of tow in what is commonly called a "dual filter". Such a filtertends to be quite expensive in comparison with conventional filter tipproduction since it necessitates the preparation of two distinct rodsand their subsequent combination into a single filter. Alternatively, ithas been suggested, as in U.S. Pat. No. 3,396,061, to merge a web ofcellulose acetate tow and a paper web just prior to forming the filter.Combined paper/acetate filters are obtainable by this process at only aminor increase over the cost required to produce conventional filters.These filters are not considered entirely satisfactory, however, sincethere tends to be nonuniform distribution of the two filter materialsacross the cross section of the filter, an undesirable arrangement sincetobacco smoke drawn through one portion of the filter will come incontact with only one filtration material while smoke drawn throughanother portion of the filter will come in contact with only the otherfilter material. As a result, no part of the smoke will be subjected tothe filtration capabilities of both materials. Moreover, the distinctlyundesirable taste produced by a paper filter is still present in adegree corresponding to the amount of paper utilized in the filter.

An alternate method of utilizing cellulose acetate staple fiber in awholly cellulose acetate filter has been the orientation of fiberdirection in the filter structure perpendicular to the flow of thematerial being filtered. Such fiber structures, however, whether in theform of nonwoven webs, felted batts or cylindrical rods lack dimensionalstability and necessitates the use of a binder to maintain the filamentsin a desired array. Moreover, such structures lack the efficiencies ofstandard paper filters because the surface area of staple fibers isbelow that of paper fibers. In addition, the use of binders presentsfurther problems since they decrease the surface area available forfiltration, add an undesirable taste to the filtered smoke and representa limiting factor in the speed of filter manufacture because of the timenecessary to attain complete bonding.

It is therefore an object of this invention to provide a sheet-likestructure with stable fiber orientation having high surface area andhaving utility as a filter material.

It is a further object of this invention to provide a process for thepreparation of materials suitable for being formulated into a highsurface area binder-free sheet-like structure.

It is an additional object of this invention to provide a process forthe preparation of a high surface area binder-free sheet-like structurewith stable fiber orientation having utility as a filter material.

It is still another object of this invention to provide a high surfacearea filter rod subdividable into cigarette filters.

These and other objects of the invention will be more apparent from thefollowing detailed description.

In accordance with this invention there is provided a high surface areacohesive, nonwoven cellulose ester fibrous sheet-like structure whichmaintains its fiber orientation in the absence of a binder and whichexhibits high filtration efficiencies at an acceptable draw. This filtermaterial comprises a web of cellulose ester staple fibers and from about5 to about 35 percent, based on the weight of staple, of a voidy, highsurface area, fibrillar cellulose ester material termed "fibrets".

The term "fibret" as employed herein identifies a high surface area,cellulose ester fibrillar material having surface areas in excess of 5.0square meters per gram, lengths of less than 1000 microns and diametersof from about 0.5 to 50 microns.

The term "binder" as employed herein identifies nonsolvent materialswhich have the ability to bond fibrous materials by forming a foreigninterface on said fibrous materials. Specifically excluded from thedefinition of the term "binder" are substances which are partialsolvents for the fibrous materials and which are more properlyidentified as plasticizers such as for instance triacetin, triethyleneglycol diacetate and mixtures containing these and other plasticizeradditives, i.e. polyethylene glycol and triacetin blends.

The phrase "high surface area" as employed herein identifies materialshaving a surface area in excess of one square meter per gram. Surfaceareas may be placed in proper perspective by noting that paper suitablefor filter applications has a surface area of 1.2 to 3.2 square metersper gram while 1/4 inch cellulose acetate staple having a denier perfilament of 1.8, 3.0 and 8.0 has a surface area of 0.35, 0.26 and 0.20square meters per gram respectively.

The staple employed in the preparation of the web is desirably celluloseester staple fiber of the conventional type having a fiber length offrom about 1/8 to 5/8 inch and a diameter per filament of from about 1.0to 8.0. It is preferred that the staple have a length of from 1/4 to 3/8inch and a denier per filament of from about 1.0 to 3.0. The sheet-likematerial is wet-laid from an aqueous slurry of cellulose ester staplefibers and cellulose ester fibrets, the process employing conventionalwet lay nonwoven apparatus. The sheet-like material preferably has asheet weight of from 20 to 40 grams per square meter, a surface area inexcess of 1 square meter per gram and a sheet breaking strength of from200 to 1000 g/5 cm. The sheet-like material is preferably corrugatedprior to being formed into cigarette rod material. The rod, when cutinto lengths suitable for a tobacco smoke filter, exhibits a draw,measured as the air pressure drop across the filter, in the range of 30to 200 millimeters of water at a flow of 17.5 ml/second for a 20millimeter length of filter.

The invention may be more readily understood by reference to thedrawings wherein:

FIG. 1 is a schematic drawing of one system suitable for the preparationof fibrets suitable for use in this invention.

FIG. 2 is a flow sheet of a process for the preparation of thesheet-like structure of this invention.

FIG. 3 is a vertical projection of an arrangement for crimping thesheet-like material of this invention and shaping the crimped materialinto a cylindrical filter rod.

FIG. 4 is a photomicrograph of the cellulose acetate fibret of thisinvention.

FIG. 5 is a photomicrograph of the sheet-like material of thisinvention.

Turning to FIG. 1, a schematic drawing is set forth illustrating thepreparation of a highly fibrillated acetate fiber. As indicated in thedrawing, a supply of cellulose acetate dissolved in acetone, oralternatively in acetic acid, is pumped through a capillary needle. Theend of the needle is situated in the throat of a venturi tube throughwhich a coagulation liquid, usually hot or cold water, is passed. Thehigh velocity of the water stream in the throat region serves toattenuate the dope stream and additionally extracts the dope solvent,thereby forming a fibret. By changing the dope concentration, waterflow, water temperature or by adding other solvents to the water stream,the size, degree of fibrillation and length of the fibret is controlled.Specific apparatus and processing conditions for preparing fibretsaccording to this method are set forth in Example 2 which follows. Theapparatus of Example 2 is also a venturi type apparatus; however, thedope is extruded above the throat of the venturi. Representative of thefibrets obtained by said Example 2 are the fibrets illustrated in FIG. 4of the drawings which is a photomicrograph magnified 600 times. Fibretsobtained by this process or by processes well known to the prior art maythen be converted into a sheet-like material as set forth in FIG. 2 ofthe drawings.

In FIG. 2 of the drawings, a flow sheet is set forth which isillustrative of a typical process employing wet lay nonwoven machineryfor the preparation of a sheet-like material of this invention. As canbe seen from the flow sheet, a slurry of fibrets is fed from a fibretsupply through a beater into a stock tank where a slurry of staple fiberis added from the staple supply. At the mixer, the resulting fluid massis agitated to provide uniform dispersion of solids and the amount ofliquid present is adjusted. The mixer feeds the head box of theFourdrinier machine wherein the water leaf is laid down, progressingthereafter through the drier and finally to the product reel.Representative of the sheet-like product obtained by this process is theproduct illustrated in FIG. 5 of the drawings which is a photomicrographmagnified 100 times. It should be noted, in reference to FIG. 5, thatthe fibrets are entangling and interlocking with themselves and thestaple fibers to create a physical bonding. The bonding effect isillustrated by the fact that similarly prepared nonwoven sheets made ofstaple fibers only contain substantially no tear strength.

The sheet-like product from the product reel is then processed by meansof the arrangement shown in FIG. 3. A sheet-like web 2 cut to anappropriate width is passed from supply roll 1 into the nip of a pair ofdriven corrugating rolls 3, corrugating rolls 3 being designated such asto produce folds and grooves and partial tears longitudinal to thedirection of travel of sheet-like web 2. Static eliminators 4 arepositioned down stream of corrugating rolls 3 so as to inhibit thelicking up of sheet-like web 2 on either of the corrugating rolls 3. Thecorrugated sheet-like material 2 is then passed over a crowned roll 5and then into the inlet funnel 6 of a rod making machine together withwrapping strip 7 which is supplied by supply roll 8. The transverselygathered and longitudinally grooved sheet-like material cylindricallywrapped emerges from funnel 6, the proud edge 9 of which is suppliedwith an adhesive by means of applicator device 10, the glued rod thenbeing passed through heated molding tool 11 so that a finished filterrod material 12 is formed. The entire rod making machine apparatus isconveniently driven by means of an endless conveyor belt 13 which isdriven by means of drive rolls 14. The finished rod material 12 may thenbe conveniently divided into suitable lengths which may be subsequentlycut into cigarette filter plugs.

As previously mentioned, the sheet-like material of this inventioncomprises a cohesive nonwoven web of cellulose ester staple fibers andfrom about 5 percent to about 35 percent of cellulose ester fibretsbased on the weight of the finished sheet. It is preferred, however,that the filter material comprises from about 10 percent to 20 percentof the cellulose ester fibrets. It is also further preferred that boththe staple and the fibrets be cellulose acetate. The filter material ofthis invention is further characterized by a surface area of betweenabout 1 square meter per gram and about 5 square meters per gram, aporosity ΔP through a one inch diameter circular sheet of between about1 mm and about 70 mm. at a flow rate of 200 cc per minute. It ispreferred, however, that the filter material have a surface area ofbetween about 2 square meters per gram and about 5 square meters pergram.

As previously noted, the fibrets used in the filter material of thisinvention may be produced by any of the known methods of the prior artsuch as the method disclosed in U.S. Pat. No. 3,342,921 and No.3,441,473 which consists of air spraying a dilute dope of the celluloseester into the atmosphere and then into a quench bath of water or themethod disclosed in U.S. Pat. No. 2,988,469 wherein cellulose acetatedope is extruded in a high velocity, unidirectional, free flowing jetstream of gas to produce fibers without the formation of shot. Thepreparation of the fibrets for use in the filter material of thisinvention is preferably carried out in accordance with the teachings ofthe description of FIG. 1 of the drawings or in accordance with theteachings of U.S. application Ser. No. 638,242 filed Dec. 8, 1975, whichprovides a rotary spinning process comprising: (1) forming a celluloseester dope, the dope preferably containing from 5 to 15 percent byweight cellulose ester, in a solvent containing from about 2 to 20percent by weight of a non-solvent liquid which is miscible with thesolvent for the cellulose ester and from about 80 to 98 percent byweight of a solvent for the cellulose ester which is miscible with thenon-solvent; (2) passing the cellulose ester dope by any suitable meanssuch as by pumping through a rotary union, to the extrusion orifice; (3)positioning a substantially cylindrical disk which is rotating about itsaxis in a heated precipitation bath, the bath consisting essentially ofa non-solvent for the cellulose ester and up to 10 percent by weight ofan organic solvent which is miscible with said non-solvent; (4)maintaining the precipitation bath at a temperature of from about 60degrees centigrade to a temperature below the boiling point of thenonsolvent therein, preferably a temperature of up to 95 degreescentigrade; (5) pumping additional liquid corresponding substantially incomposition with the composition of the precipitation bath into theprecipitation bath and past the periphery of the rotating disk at a flowrate of at least 0.5 liter per minute per extrusion orifice, preferablyat least 1.0 liter per minute per extrusion orifice; (6) extruding thedope through one or more orifice or capillary needles positioned on theperiphery of the rotating disk, or alternatively, positioning a wall orplate around the periphery of the rotating disk and, optionallyextruding the dope from orifices positioned on the wall or on theperiphery of the disk into the precipitation bath, the extrusion orificeor orifices having a diameter ranging from 0.005 to 0.040 inch; (7)precipitating the cellulose ester dope in the precipitation bath andremoving the precipitatate from the bath; (8) subjecting the fibretprecipitate to boiling, preferably in water, for at least 5 minutes,preferably from 10 to 30 minutes, to aid in removing residual organicsolvent, and to expand and set the voidy fibrillar structure; (9)homogenizing the fibrets and reducing the particle size of the celluloseester fibrets; and (10) the cellulose ester fibrets may optionally besubjected to a drying operation to partially or totally dry the fibretsin preparation for the production of the filter material of thisinvention.

The dope formulation contains a cellulose ester such as celluloseacetate, cellulose triacetate, cellulose acetate butyrate, benzylcellulose, or mixtures thereof. The preferred fibrets are produced fromcellulose acetate dissolved in a solvent comprising two misciblecomponents: an organic solvent such as acetone, methyl ethyl ketone,acetaldehyde or ethyl acetate and a liquid non-solvent for the celluloseester such as water, methanol or ethanol. The liquid non-solvent makesup from about 2 to 20 percent by weight of the solvent mixture. Thepreferred organic solvent is acetone which is miscible with thepreferred nonsolvent, which is water. The concentration of celluloseester in the solvent mixture should be from about 5 to 15 percent byweight, preferably 5 to 8 percent. Below about a 5 percent celluloseester level, the process is economically undesirable. Because theorganic solvent flashes off during the process and can cause variousproblems relating to solvent recovery and the presence of a flammable,volatile solvent in the atmosphere, it is desirable to keep the organicsolvent level as low as possible and still obtain the desired product.In addition, keeping the dope solids level relatively low provides adope which also has a relatively low viscosity and is much easier tohandle and extrude without significant clogging of the extrusionorifices. Mineral additives such as TiO₂, BaSO₄ and Al₂ O₃ can beincluded in the dope solution if desired. If they are present, they maybe included at levels up to 50 percent of the weight of acetate as partof the total solids in the dope and are ball milled to a fine particlesize.

The cellulose esters of this invention are preferably prepared from anacetylation grade wood pulp with higher than 90 percent by weight ofhemicellulose. However, it should be understood that lesser quality woodpulps are also acceptable; that is to say, wood pulps having ahemicellulose content of from 5 percent to 10 percent by weight.Correspondingly, it is preferred that high purity cellulose esters beemployed. Purity is equated to filterability which represents the numberof pounds of dissolved ester that can be filtered through a typicalplant first filtration medium before the medium plugs to an undesirableextent. Values of from 30 to 60 lb.ft.² of filtering area are typical ofcommercial textile grade esters. However, cellulose esters havingplugging valves of less than 30 lbs./ft.² are also suitable for purposesof the preparation of the fibrets for use herein. That is to say, thefibrets of this invention may be prepared from cellulose esters having alesser degree of purity than that which is considered acceptable forfilament-forming applications.

When fibrets for use in the filter material of this invention are madeby the spray spinning process as set forth in the description of FIG. 1of the drawings, high pressure water is preferably used as a sprayingmedium. By using water, a smooth running nonplugging spray process isachieved, and the dimensions of the precipitated product appear to befiner than that product obtained by following the teachings of U.S.patent application Ser. No. 638,242 filed Dec. 8, 1975. By changingwater flow and temperature, the length of the fibrets can be altered.Cold water and/or high flow rates will minimize fibret length. The dopesolution preferably consists of 10 percent solids dissolved in a 90percent acetone, 10 percent water solvent. Solids concentrations between5 percent and 12 percent can be utilized with higher concentrationsgiving a coarser stringy material and lower concentrations beingeconomically undesirable. The solvent mixture can range from 100 percentacetone to 60 percent acetone, 40 percent water with little effect onthe product properties. It should be understood, however, that any ofthe cellulose ester dope formulations which have previously been setforth as suitable for use in the process of U.S. application Ser. No.638,242 filed Dec. 8, 1975, are also suitable for use in the sprayspinning process set forth in the description of FIG. 1 of the drawings.Mineral tracers such as TiO₂, BaSO₄ and Al₂ O₃ may also be included inthe dope solutions if desired.

The preferred fibrets of this invention have an extremely large surfacearea per unit of weight. Whereas ordinary cellulose acetate filamentshave a surface area of about 0.25 square meters per gram, the celluloseester fibrets used in the production of the filter material of thisinvention have surface areas in the range of from about 12 to 25 squaremeters per gram in most instances, but substantially always have surfaceareas of greater than 5.0 square meters per gram.

As previously noted, cellulose ester staple fiber employed is desirablymaterial of the conventional type having a fiber length of from about1/8 to 5/8 inch and a denier per filament of from about 1.0 to 8.0. Itis preferred that the staple have a length of from 0.25 to 0.375 inchand a denier per filament of from about 1.4 to 3.0. The fibercross-section may be the normal crinulated form produced by extrusionthrough a round orifice or have other cross sections produced byextrusion through non-circular orifices, i.e., Y, X or dog bonecross-sections and the like. The cellulose ester staple may be one ormore selected from the group of cellulose acetate, cellulose propionate,cellulose butyrate, cellulose benzoate, cellulose acetate formate,cellulose acetate propionate, cellulose acetate butyrate and the like.The esters may be ripened and acetone soluble, such as conventionalcellulose acetate, or may be substantially fully esterified, i.e.,contain fewer than 0.29 free hydroxyl groups per anhydroglucose unit,such as cellulose triacetate. The preferred cellulose ester staple iscellulose acetate.

As discussed in conjunction with FIG. 2 of the drawings, the sheet-likematerial of this invention may be prepared from a slurry formed of thecellulose ester staple fibers and the cellulose ester fibrets in water.Between 5 and 35 percent, and preferably between 10 and 20 percent,fibrets should be present based on the weight of fibrous material used.Solids should comprise between 0.005 and 0.1 percent, and preferablybetween 0.01 and 0.03 percent, of the slurry.

The staple fibers and fibrets should be thoroughly mixed and uniformlydistributed throughout the slurry. This may be accomplished by stirringor mixing either manually or with any conventional mixing apparatus. Thestaple fibers and fibrets may be added individually to the water andblended. However, when the fibrets have been prepared utilizing water asthe boiling medium to expand and set the voidy fibrillar structure, thefibret/water cake, with additional water if necessary, may be used as abase for the staple fiber/fibret/water slurry which, in this case, wouldbe prepared merely by the addition of the appropriate amount of staplefibers to the fibret/water blend.

As previously mentioned, the slurry is deposited on conventionalpaper-making apparatus to form a sheet-like material which has utilityas a filter material such as for instance in sheet form for use in facemasks and respirators or in corrugated and condensed form for use as acigarette filter. Cigarette filter rods produced from corrugated filtermaterial of this invention exhibit equal or higher filtrationefficiencies at a given pressure drop than rods of similarly corrugatedpaper webs with significantly improved taste.

The invention is further illustrated by the following examples whereinall parts and percentages are by weight and all temperatures are indegrees centigrade, unless otherwise specified.

EXAMPLE 1

To prepare the rotary spun fibrets of the sheet-like material of thisinvention, a 7.5% solids dope formulation containing fiber gradecellulose acetate having an acetyl value of abot 55 is preparedaccording to the following formulation:

    ______________________________________                                                       Parts by Weight                                                ______________________________________                                        Cellulose acetate flake                                                                        90.5                                                         Acetone          1080.0                                                       Water            120.0                                                        ______________________________________                                    

by first mixing the acetone and water and then adding the acetate flake.The mixture is gently tumbled until the cellulose acetate is completelydissolved. 20 percent by weight of TiO₂ tracer material is then addedbased on the solids weight of the dope. Utilizing the apparatusdisclosed in copending U.S. application Ser. No. 608,416, the dope isplaced in a storage tank and then pumped through a conduit by a gearpump to a hollow shaft through a rotary union into the interior of a sixinch hollow disk rotating at 2900 revolutions per minute (a peripheralspeed of 1390 meters per minute). The disk is immersed in aprecipitation bath for the dope which consisted essentially of waterheated to a temperature between 75 and 85 degrees centigrade. The hollowrotating disk has three 0.014 inch diameter orifices on the peripheralsurface of the disk. The disk rotates within an annular wall or ringspaced about 3/16 inch from its peripheral surface. Water maintained ata temperature between 75 and 85 degrees centigrade is pumped into theprecipitation tank and passed through the annular space between theperiphery of the disk and the wall at a flow rate of 6.7liters/minute/orifice. Short, voidy fibers having a high degree offibrillation are generated by the rapid precipitation of the celluloseacetate and the shear and high draw on the dope stream issuing from theextrusion orifices.

The material is then swept to the surface of the precipitation bath andoverflows onto a collection screen where a portion of the water andacetone is separated from the fibrillar material. The collected materialis then boiled for about 20 minutes at a pressure of 15 p.s.i.g. isremove residual solvents and harden the voidy structure. The fibrillarmaterial is then redispersed in water and passed to a Gaulin 15 Mhomogenizer manufactured by Gaulin Corporation, Everett, Mass., wherethe fibret lengths are reduced at a pressure to 3000 p.s.i. to about 500microns or less. The fibrillar material is then suction-filtered toprovide a cake containing about 12 percent by weight cellulose acetatefibrets and 88 percent water. The fibrets as they are removed from theprecipitation bath have a relatively limp structure and relatively shortaverage fiber lengths. After homogenization and heat treatment, thefibrets are no longer limp but rather are set and with a somewhatexpanded voidy structure. The fibrets are generally irregular in shapehaving a length varying from about 1 to about 500 microns and a diameterfrom less than 1 up to about 50 microns. The fibrets and 1/4 inch stapleof 1.8, 3.0, and 8.0 dpf (Y cross section) were formed into sheets on alaboratory nonwoven sheet-forming apparatus. The fibrets were thendispersed to 0.5 percent concentration in a Waring blender, and thisslurry was further dispersed in 250 gallons of water containing 227grams of staple. This slurry was pumped onto a 60×40 mesh brass screenmoving at 5.5 feet per minute. Vacuum was applied to the screen (8-10inches of mercury) to remove water and the formed sheet was transferredto a felt belt and finally to two steam drying cans containing steam at20 and 30 pounds pressure. The finished 12 inch wide sheets were 0.0037to 0.0038 inch thick and had properties as set forth in the followingtable designated as Table I:

                  TABLE I                                                         ______________________________________                                                                   Percent                                                   Fibrets             Fibrets                                            Staple Added    Sheet Weight                                                                             Finished                                                                             Breaking Strength                           dpf    to Slurry                                                                              g/m.sup.2  Sheet  g/5 cm strip                                ______________________________________                                        1.8    101 g.   37.3       16.5   300                                         3.0    216 g.   33.0       17.6   459                                         8.0    309 g.   32.1       20.1   755                                         ______________________________________                                    

Strips were cut from these sheets at widths indicated below and passedthrough heated corrugating rolls containing 25 teeth per inch with asurface temperature of approximately 120 degrees centigrade. Thecorrugated strips were gathered into a garniture and wrapped with paperto form filter rods of 90 mm length and 25 mm circumference. These rodswere cut into 20 mm tips which were attached to 65 mm tobacco columnsfor measurement of the removal efficiency (% removed by filter) of totalparticulate matter (SRE), Nicotine alkaloids (NRE), and "tar" as definedby the Federal Trade Commission (TRE). The results were as set forth inthe following table designated as Table II:

                  TABLE II                                                        ______________________________________                                                              Avg. Tip  Removal                                       Strip       20 mm     Pressure  Efficiency                                    Width       Tip Weight                                                                              Drop      at 60 mm PD                                   Fiber dpf                                                                            (inches) (mg.)     (mm H.sub.2 O)                                                                        SRE  NRE  TRE                               ______________________________________                                        1.8    7.5      .118      43      68.0 65.7 64.9                              3.0    9.0      .167      62      62.1 58.8 57.2                              8.0    10.0     .204      70      62.5 56.7 57.5                              ______________________________________                                    

The sheets containing low dpf fibers appear to be better on severalcounts. The retention of fibrets in the sheet-making process isimproved, and a weaker, more readily corrugated sheet is formed. Theresulting tip weights and pressure drops are reduced and, in the case of1.8 dpf, a considerable increase in removal efficiency is achieved.

EXAMPLE 2

To prepare the spray spun fibrets of the filter material of thisinvention, the dope formulation of Example 1 is again employed.Utilizing a nozzle and cap spray apparatus as prepared by SprayingSystems Company Set-up #22B, 3201 Randolph Street, Bellwood, Ill.,60104, the dope is placed in a storage tank and then pumped through acentrally positioned 0.40 inch extrusion nozzle at a rate of 420 gramsper minute. Precipitating and attenuating water at 60 to 65 degreescentigrade is pumped through the three orifices surrounding theextrusion nozzle at a rate of 9 to 10 liters per minute at a pressure of180 pounds per square inch. The dope-water mixture exits through a 0.110inch orifice located 0.140 inch away from the dope nozzle into a tubefilled with water where the fibrets are precipitated. The fibrets arethen collected and purified and formulated into paper by the sameprocess as is set forth in Example 1, with the exception that thematerial was boiled at atmospheric pressure and homogenization wasomitted.

EXAMPLE 3

The process of Example 2 was repeated with the exception that the sprayspun fibret material was subjected to an atmospheric boiling operationand then passed through a Gaulin homogenizer.

EXAMPLE 4

The process of Example 2 was repeated with the exception that the sprayspun material was boiled under pressure at 120 degrees centigrade andthen subjected to homogenization by passage through a Gaulinhomogenizer.

The samples from Examples 2, 3 and 4 were formulated into sheet-likewebs and evaluated in the following table designated as Table III:

                  TABLE III                                                       ______________________________________                                                                  **          Smoke                                           Sheet             Breaking                                                                             Tip  Removal                                         Wt.               Strength                                                                             Wt.  Efficiency                              Treatment                                                                             g/m.sup.2                                                                             % Fibrets g/5 cm (g)  at 50 mm PD*                            ______________________________________                                        Example 2                                                                             32.1    24.3      1300   .181 54                                      Example 3                                                                             37.1    16.0      1300   .182 55                                      Example 4                                                                             32.1    23.6      1018   .195 58                                      ______________________________________                                         *interpolated or extrapolated from data at higher and lower pressure drop     **The high sheet strengths resulted from higher temperature drying with       steam can pressures of 35 and 45 p.s.i.                                  

As can be interpreted from the data of Table III, boiling under pressureand homogenization provide a finer material which gives a weaker, morereadily corrugated sheet, which in turn improves the smoke removalefficiency at a given pressure drop.

In order to determine the effect of a fibret level in a sheet-likematerial produced according to the teachings of this invention, samplesof the rotary spun fibrets of Example 1 and the spray spun fibrets ofExample 4 were combined with staple and wood pulp fibers to form sheetscontaining between 5 and 100 percent fibrets. The pertinent data isreported in the following table designated as Table IV:

                                      TABLE IV                                    __________________________________________________________________________    Fibret             Sheet                                                                              Tip      Removal                                      Level                                                                             Staple   Spinning                                                                            Strength                                                                           Wt.                                                                              Tip P.D.                                                                            Efficiencies                                 (%) (dpf × length(in))                                                               Technique                                                                           (g/5 cm)                                                                           (g)                                                                              (mm H.sub.2 O)                                                                      SRE                                                                              NRE                                                                              TRE                                    __________________________________________________________________________      5*                                                                              1.8 × 1/4                                                                        Spray 182  .115                                                                             60    71.1                                                                             63.7                                                                             68.0                                   11.9                                                                              3.0 × 1/4                                                                        Rotary                                                                              228  .121                                                                             58    62.2                                                                             60.0                                                                             59.5                                   15.8                                                                              3.0 × 1/4                                                                        Rotary                                                                              354  .119                                                                             58    62.6                                                                             59.7                                                                             59.8                                   17.6                                                                              3.0 × 1/4                                                                        Rotary                                                                              459  .160                                                                             60    62.1                                                                             58.8                                                                             57.2                                   23.1                                                                              3.0 × 1/4                                                                        Rotary                                                                              505  .127                                                                             60    62.0                                                                             61.1                                                                             59.5                                   27.7                                                                              1.8 × 1/4                                                                        Rotary                                                                              771  .124                                                                             60    63.0                                                                             57.5                                                                             61.4                                   30* 1.8 × 1/4                                                                        Spray 767  .114                                                                             60    58.3                                                                             50.9                                                                             51.3                                   50* .sup.+   Rotary                                                                              --   .168                                                                             56    54.2                                                                             50.0                                                                             49.0                                   75* .sup.+   Rotary                                                                              --   .222                                                                             60    43.2                                                                             32.3                                                                             37.8                                   100*                                                                              --       Rotary                                                                              --   .230                                                                             56    37.2                                                                             27.2                                                                             29.5                                   __________________________________________________________________________     *prepared on Noble and Woods hand sheet apparatus, fibret levels estimate     from fibret concentration in slurry and retention on the forming screen       .sup.+ wood pulp substituted for staple                                  

As can be seen from Table IV, the data illustrates that fibret level iscorrelated with sheet strength but does not greatly affect the smokeremoval efficiencies at levels below 30 percent. When fibrets arepresent at levels of 30 percent or more, the sheet strength issufficiently high that the corrugation process does not open thestructure up sufficiently to make a good filter. Consequently, there isa progressive loss in removal efficiency as the fibret level increasesabove 30 percent. There is also a pattern of increasing tip weight asthe fibret level increases, which would make high levels undesirablefrom an economic viewpoint. At the low fibret level of 5 percent, thesmoke removal efficiency values are high, partially because of the lowerdenier per filament staple utilized and partially because of theopenness of the corrugated sheet. The sheet strength is quite low andthe material is difficult to handle in sheet and rod making because ofthis.

EXAMPLE 5

3 denier per filament 1/4 inch cellulose acetate staple was employedtogether with the cellulose acetate fibrets of Example 4 to prepare asheet-like material. A 12 inch width laboratory fourdrinier machinehaving a 90×100 mesh screen was used and the drying conditions were suchthat unglazed sheets were obtained (10 pounds per square inch stream canpressures on dryer rolls). The sheet-like material produced was thenconverted into cigarette filters according to the method set forth inthe description of FIG. 3 of the drawings. The data from this example isset forth in the following table designated as Table V:

                  TABLE V                                                         ______________________________________                                                              Removal                                                 Sheet         Breaking Pressure                                                                              Tip  Efficiency                                 Wt.  Fibrets Strength Drop    Wt.  SRE  NRE  TRE                             (g/m.sup.2)                                                                         (%)     (g/5 cm) (mm H.sub.2 O)                                                                        (g)  (%)  (%)  (%)                             ______________________________________                                        20.0  13.6    100      39.5    .126 54.8 50.0 53.1                            35.5  11.9    228      40.5    .116 57.2 51.5 54.0                            41.1  13.2    325      40      .139 56.6 50.0 52.0                            ______________________________________                                    

EXAMPLE 6

1.8 denier per filament, 1/4 inch cellulose acetate staple was employedtogether with the cellulose acetate fibrets of Example 4 to prepare asheet-like material. The staple and fibret levels in the initial slurrywere reduced to 113 and 32 grams per 250 gallons of water respectively.A 12 inch width laboratory fourdrinier machine having a 90×100 meshscreen was used and the drying conditions were such that unglazed sheetswere obtained (10 pounds per square inch steam can pressures on dryerrolls). The data from this example is set forth in the following tabledesignated as Table VI:

                  TABLE VI                                                        ______________________________________                                                                          Removal                                     Sheet         Avg.     Avg.       Efficiency                                  Wt.   Fibrets Tip Wt.  Pressure Drop                                                                            at 60 mm P.D.                               (g/m.sup.2)                                                                         (%)     (g)      (mm H.sub.2 O)                                                                           SRE  NRE  TRE                               ______________________________________                                        26.8  17.3    .148     52         71.0 66.5 68.4                              37.3  16.5    .118     43         68.0 65.7 64.9                              ______________________________________                                    

The data from Tables V and VI indicate that there is an optimum sheetweight for filters produced from these sheet-like materials in theregion of 26 to 36 grams per square meter. It should be understood ofcourse that optimum sheet weight will vary depending on other physicalparameters of the sheet.

In order to determine the efficiency of the cigarette filter rodproduced from the sheet-like web of the instant invention in comparisonwith filter rod materials of the prior art, a sheet-like web wasprepared according to the following example designated as Example 7:

EXAMPLE 7

The process of Example 6 was repeated employing sufficient fibrets toproduce a sheet containing 17.3% fibrets and a sheet weight of 26.8grams per square meter. The sheet was then corrugated in thelongitudinal direction and hand rolled into a cigarette filter. Thefilter was then evaluated against commercially available filters. Thepertinent data is set forth in the following table designated as TableVII:

                  TABLE VII                                                       ______________________________________                                                                           Smoke                                                                 Filter  Removal                                                   Pressure Drop                                                                             Wt.     Efficiency                                 Filter Construction                                                                          (mm H.sub.2 O)                                                                            (g)     (%)                                        ______________________________________                                        Acetate tow, 1.8 dpf,                                                         43000 total denier                                                                           80          .116    56.5                                       Acetate tow, 3.3 dpf,                                                         44000 T.D.     77          .138    53.0                                       Corrugated cellulose                                                          paper          80          .208    70.7                                       Corrugated acetate                                                            non-woven (17.3% fibrets)                                                                    80          .158    77.6                                       ______________________________________                                    

As can be seen from the data of Table VII, it is evident that thecorrugated acetate sheet-like material has superior smoke removalperformance to other structures and hence is the preferred construction.Where, nowever, for various reasons, such as smokers' preference, it isdesired to alter the fabrication of the filter or reduce its filtrationefficiency, the sheet-like material may be shredded or needle punchedrather than corrugated prior to being rolled into a filter.Alternatively, wood pulp may be blended into the cellulose fibret andcellulose staple mix employed in the preparation of the sheet-likematerial of this invention. In order to domonstrate the aforementionedreduction in filter efficiency, the sheet-like material of Example VIIwas needle punched prior to hand rolling, shredded prior to hand rollingand adulterated with wood pulp, the pertinent data being as set forth inthe following table designated as Table VIII:

                  TABLE VIII                                                      ______________________________________                                                         Pressure  Filter  Smoke                                                       Drop      Wt.,    Removal                                    Filter Construction                                                                            (mm H.sub.2 O)                                                                          (g)     Efficiency                                 ______________________________________                                        Needle punched acetate                                                        non-woven        81        .126    69.5                                       Shredded acetate non-                                                         woven corrugated 81        .138    67.0                                       1.8 dpf acetate-wood pulp                                                     (17.5%) non-woven                                                                              80        .147    66.0                                       ______________________________________                                    

The most preferred means for adjusting the smoke removal efficiency ofthe sheet-like material of this invention is layering of the corrugatedsheet-like material with cellulose acetate tow prior to forming filterrods. The preparation of such layered filters is fully set forth in U.S.Pat. No. 3,396,061. However, it is preferred to simultaneously corrugatethe layered structure. When, for instance, the corrugated sheet-likematerial of Example VII is layered and simultaneously corrugated with3.3 denier per filament cellulose acetate tow on a 50/50 weight bases,it is found that a cigarette filter weighing 0.154 grams having apressure drop of 78 mm H₂ O will produce a smoke removal efficiency of65.9%.

To test the performance of fibret-staple non-wovens in sheet filtrationapplications, sample sheets were prepared according to the process setforth in Example 1 except that hand sheet forming apparatus (Nobel andWoods) was employed. Acetate and polyester staple containing variouslevels of fibrets was employed in making the sheets. These were cut into13/4 diameter disks and mounted in Cambridge filter pad holders to testtheir removal efficiency for tobacco smoke, which is considered in thisinstance to be a representative aerosol in the 0.1-1.0 particle sizerange. Two types of samples were prepared, one of which consisted ofnormal thin sheets (0.1-0.2 mm thick) and the other of thick sheets(1.4-1.6 mm thick), which approximately matched the pressure drop ofcommercial face mask filtering material. In the case of the thin sheetspressure drop comparisons were made by stacking sheets in the Cambridgepad holders. The commercial materials used for comparison purposesconsisted of disks cut out of face mask materials. One consisted of a BM2166 dust and mist respirator pad made by the Mine Safety Appliances Co.of Pittsburg, Pa., and the other a bonded fibrous material used as amolded face mask labelled TC-21C-132 (No. 8710) manufactured by theMinnesota Mining and Manufacturing Co. of Minneapolis, Minn. The resultsare as given in the following table designated as Table IX:

                                      TABLE IX                                    __________________________________________________________________________                                      Pad                                                                     Pad   Smoke                                                      Staple dpf                                                                              Pad                                                                              pressure                                                                            Removal                                                 No.                                                                              &     %   wt.                                                                              drop  Efficiency                                  Pad Material                                                                              Pads                                                                             length(in)                                                                          Fibrets                                                                           (g)                                                                              (mm H.sub.2 O)                                                                      %                                           __________________________________________________________________________    thick acetate non-woven                                                                   1  8.0 × 1/4                                                                     5   .890                                                                             7     70.9                                        thick acetate non-woven                                                                   1  33 × 1/4                                                                      5   .780                                                                             11    79.2                                        thick acetate non-woven                                                                   1  3.3 × 1/4                                                                     5   .802                                                                             12 85.8                                           thin acetate non-woven                                                                    1  1.8 × 1/4                                                                     10  .091                                                                             3     31.7                                        thin acetate non-woven                                                                    3  1.8 × 1/4                                                                     10  .324                                                                             8     62.9                                        polyester staple +                                                                        1  1.5 × 1/4                                                                     10  .126                                                                             2     30.4                                        fibret non-woven                                                                          5  1.5 × 1/4                                                                     10  .626                                                                             6     77.4                                        BM 2166, #1 1  --    --  .687                                                                             3     39.5                                        BM 2166, #1 1  --    --  .695                                                                             3                                                 0.3                                                                           BM 2166, #2                                                                               -- --    695 7  44.8                                              BM 2166, #2 1  --    --  .712                                                                             9     43.3                                        7C-21C-132  1  --    --  .400                                                                             7     74.6                                        TC-21C-132  1  --    --  .400                                                                             9     68.8                                        __________________________________________________________________________

It is evident that the staple non-wovens are effective in removingtobacco smoke particles in both the tick and multiple thin padconfigurations. The non-woven pads of this invention are found to haveequal or greater efficiency than prior art materials in the highpressure drop range.

Having thus disclosed the invention, what is claimed is:
 1. As a tobaccosmoke filter precursor, a corrugated binder-free non-woven fibrouscellulose ester sheet-like material, comprising from about 65% to about95% cellulose ester staple fibers and from about 5% to about 35%precipitated cellulose ester fibrets, said fibrets having a surface areaof from 12 to 25 square meters per gram and lengths of less than 1000microns, said sheet-like material having a sheet weight of from 20 to 40grams per square meter, a surface area in excess of 1 square meter pergram, a sheet breaking strength of from 200 to 1000 g/5 cm. and aporosity through a 1 inch diameter circular sheet of said non-wovenfibrous cellulose ester material of between about 1 millimeter and 70millimeters at a flow rate of 200 cc. per minute.
 2. The filtersheet-like material of claim 1 wherein the fibrets comprise celluloseacetate.
 3. The filter sheet-like material of claim 1 comprising fromabout 10% to about 25% cellulose ester fibrets.
 4. The filter sheet-likematerial of claim 1 wherein the cellulose ester staple fiber is selectedfrom the group consisting of cellulose acetate, cellulose propionate,cellulose butyrate, cellulose benzoate, cellulose acetate formate,cellulose acetate propionate, cellulose acetate butyrate, and mixturesthereof.
 5. The filter sheet-like material of claim 1 wherein thecellulose ester staple fiber comprises cellulose acetate.
 6. A method ofproducing a self-supporting, non-woven, binder-free cellulose esterfibrous tobacco smoke filter precursor corrugated sheet-like materialwhich comprises preparing a slurry of cellulose ester staple fibers andcellulose ester fibrets in water, said fibrets being precipitatedfibrets having a surface area of from 12 to 25 square meters per gramand lengths of less than 1000 microns, mixing the slurry to produce ahomogenous mixture of staple fibers and fibrets, depositing the slurryon a porous surface which permits the drainage of water from thedeposited fibrous structure, drying the fibrous structure, removing thethus-formed sheet-like material from the porous surface, and thencorrugating the sheet-like material whereby a corrugated sheet-likematerial is obtained having a sheet weight of from 20 to 40 grams persquare meter, a surface area in excess of 1 square meter per gram, asheet breaking strength of from 200 to 1000 g/5 cm. and a porositythrough a 1 inch diameter circular sheet of said material or betweenabout 1 millimeter and 70 millimeters at a flow rate of 200 cc. perminute.
 7. The method of claim 6 wherein the aqueous slurry containsfibrous material comprising from about 65 percent to about 95 percentcellulose ester staple fiber and from about 5 percent to about 35percent cellulose ester fibrets.
 8. The method of claim 6 wherein thecellulose ester staple fibers have a denier per filament of from about1.05 to about 8.0 and a length of from 1/8 inch to about 5/8 inch. 9.The filter sheet-like material of claim 1 wherein said surface area isbetween about 2 square meters per gram and about 5 square meters pergram.